The present invention relates to an extrusion die. More specifically, the invention is directed to an extrusion die, of the pipe die type, which is particularly adapted to the extrusion of polymeric materials having high melting points.
Apparatus for the extrusion of thermoplastic pellets or granules is widely known. Generally, such apparatus includes an extruder adapted to feed a thermoplastic material in the molten state to an extruding die having a plurality of nozzles opening from a nozzle plate. As the thermoplastic material is extruded through the die nozzle in the form of rods or threads, the latter are cut transversely into small generally cylindrical fragments by means of a rotary cutter, e.g. knife or blade, which is adapted to rotate in sliding relation with the face of the nozzle plate.
It is generally necessary to heat the die in an attempt to obtain uniform heat distribution to each extrusion passageway to prevent the passageways from becoming plugged. If, for example, the temperature through any passageway(s) should drop below the desired critical temperature applicable to a particular thermoplastic material being extruded, the material may plug that passageway(s) and cause the remaining nozzles to extrude at an accelerated rate, thereby affecting the uniformity of the pellets formed by action of the rotating blades or knives. Accordingly, it is common practice to provide a heating jacket or cavity around the tubes or pipes which form the passageways through which the molten thermoplastic passes from the extruder to the nozzle plate. Circulated through this cavity or heating jacket is a heating medium, e.g. steam, whereby the tubes or pipes, and hence the thermoplastic material, are generally kept at a uniform, desired temperature, well above the point at which the thermoplastic materials will solidify. The problem of heat transfer to the tube is particular important when extruding thermoplastic materials having a high melt index. Such materials, for example some polyethylene, most polypropylenes and numerous other thermoplastic resins, have intrinsically high melting points. Accordingly, it is absolutely necessary that the tubes be kept at the desired temperature to prevent solidification.
In typical prior art extrusion dies of the tube or pipe type, such as for example that shown in U.S. Pat. No. 4,327,050, the tubes or pipes which form the extrusion passageway are received in bores in the die body and are in turn welded to the back side of a die plate member on the face of which is carried the nozzle plate. Generally, the tubes or pipes are also welded to the die body. In part, the welding between the tubes and the die plate member and the tubes and the die body seals the passageways in the tubes and die plate from the heating jacket such that (1) the heating medium in the heating jacket does not escape and mix with the extruding polymer and/or (2) the polymer is not forced into the cavity. Such leakage between the cavity and the tubes can cause the nozzles to extrude at varying rates, thereby affecting the uniformity of the pellets being formed by action of the rotating knives.
The welding of the tubes or pipes to the die plate member presents several difficulties. When in operation, the pressure internally of the tubes and die plate member is relatively high in order to form the thermoplastic material out through the nozzle plate. The temperature, both internally of the tubes and, of necessity, in the heating jacket surrounding the tubes, is also quite high. Because of the welding process, there remains even after post-weld heat treatment, a "heat affected zone" surrounding each tube. The combination of relatively cold water (140.degree. F.) on the die face, high temperature heating media and high temperature polymer, pressure (3000 psi), causes the die body to flex. This distortion affects welded areas close to the die face which is closest to the cooler temperature water. This constant interaction of pressure and temperature gradients causes fatigue cracks in the welded tubes.
Additionally, any welded area has a slightly altered metallurgical chemistry due to different cooling rates in the weld puddle and depletion of the alloying elements caused by the heat of fusion. This altered chemistry is susceptible to corrosive attack by the cooling water and product impurities. Accordingly, stress corrosion cracking is often evident around the tubes welded in the die body.